A program shutter whose blades are driven by a stepper motor operates in the manner described below. When a shutter-release button is depressed, pulses of a certain frequency are applied to the stepper motor to rotate it in the forward direction for gradually opening the sectors. When the motor rotates through an angle corresponding to the light exposure value, the phase of the pulses is switched to another phase. Then, the motor is rotated in the backward direction to close the sectors thereby obtaining an appropriate exposure
The shutter constructed as described above has some drawbacks. In particular, when the shutter is at rest, the members that open and close the sectors are held at their home positions by springs to prevent the occurrence of a malfunction. Therefore, when the sectors are being opened, the motor is loaded by the springs. Also, during the opening process of the sectors, the diameter of the opening does not change uniformly with time because of either the inertia the of the movable components or the fit tolerances of various components. This has made it impossible to accurately control the exposure